It depends on your car rental company, but the information in this reply is typical for airport rentals with major companies.
Keeping the car a few extra hours costs nothing unless your car goes into another 24-hour day. In other words, if your rental began at 5 p.m. and your current reservation calls for it to be back at 2 p.m. (because the flight you expected to take originally left around that time), you can actually return your car at 5 p.m. without going into another 24-hour day.
Also, most car rental companies have a grace period, perhaps 30 minutes (varies by company). You can go slightly into the next 24-hour day without incurring a charge.
If you go over a 24-hour day mark beyond any grace period, the hourly rate kicks in. The hourly rate is typically half the daily rate, but only up to the daily rate. In other words, if your daily rate is $40, but you keep the car an extra hour, you pay an extra $20; if you keep the car two extra hours, you pay an extra $40 -- but you've then paid for an extra day.
It might be worth an extra day's rental fee to have another 3 1/2 hours of vacation time. Or it might be better to get to the airport early and apply that money to a meal there.
However, returning your car after you said you would return it can cause problems:
Some rates, especially weekend rates, are only good for rentals that begin and end within a certain window. For example, some weekend rates become invalid if the car is returned after 12 noon on Monday, so you then pay a higher rate structure for the entire rental.
At some point, your car is considered late. And at some point, it's considered stolen. With the corporate rate I use, I think I can keep the car up to 24 hours without even having to call the car rental company, but your car rental company or rate may be more stringent. It would be a good idea to call your car rental company.
Pre-paid car rentals booked through an online discount site can have ridiculous rate rules (with hight costs) if you don't return the car when you're supposed to. That seems to one way the car rental companies can offer such low rates through such channels.
